This invention relates to a continuous concentrating and denitrating apparatus using microwave heating, which is capable of the continuous treatment of the concentration by evaporation and denitration of a nitrate solution of plutonium, uranium, a mixture thereof or the like.
Heat treatment system using microwave heating has been proposed for the evaporation and direct denitration of a nitrate solutin of plutonium, uranium or a mixture thereof. (see, for example, U.S. Pat. No. 4,364,859). Denitration by microwave heating enables the production of a good oxide powder of small, spherical particle sizes, so that this technique has come to be used lately instead of a conventional fluidized bed method in processes for producing oxide powders for nuclear fuel pellets from nitrate solutions obtained by the reprocessing of spent nuclear fuel.
However, the method using microwave heating employs a batch-type treatment system in which a heating vessel is installed in an oven to which microwave is applied, and the nitrate solution to be treated is poured into the heating vessel, is evaporated by microwave heating, and is then denitrated. The term "denitration" means that the nitrate solution is thermally decomposed at final stage and the nitric acid is driven away. Therefore, it is necessary with such method to stop the application of microwave, remove the produced denitrated powder, and feed the next batch of nitrate solution to be treated into the heating vessel upon the completion of each run, so that the operation is troublesome and poor in efficiency. Also, since the nitrate solution being treated includes nuclear fuel materials which could reach a critical state, such as plutonium, appropriate control of criticality for safety is required during the treatment, and thus there are placed restrictions on the quantity of nitrate solution that can be fed to the heating vessel during one run. Therefore, such batch-type method is difficult to provide bulk treatment.
When the nitrate solution must be treated, not on a laboratory scale, but in great quantities, a two-stage process has been employed in which the nitrate solution being treated is first concentrated to a fairly high degree in an evaporation concentrator, and then the concentrated solution is transferred to a microwave denitration device and is denitrated therein to form the corresponding denitrated oxide powder. Steam is usually used as the heat source in the evaporation concentrator of a shell-and-tube type, the nitrate solution being treated is passed through the tube side in the concentrator while steam is passed through the shell side, so that heat is exchanged between them to rise the temperature of the solution. This system has the likelihood of fire hazard as oil mixed into the solution could ignite if the steam temperature rises to increase the solution temperature to an abnormal level. In order to prevent such a hazard, a device controlling the steam temperature must be provided to keep the temperature of the solution-heating tube wall below a predetermined level.
Thus the prior art technique described above has many problems. Considering, for example, the evaporation concentrator alone, since the heat is exchanged between two fluids across the tube wall when steam is used for heating, it is necessary to provide a heat exchanger with a structure in which there is absolutely no leakage of either fluid, and consider environmental factors during the use thereof, such as its corrosion resistance, radiation resistance, etc. This means that a great deal of technical effort and a huge cost are required for the selection of material of this device, decisions on its construction, welding techniques and the inspection of welded portions, checks on its air-tightness and lack of leakage, and so forth.
Even if an electric heater such as, for example, a sheathed heater is used instead of steam for heating, it is necessary to provide several devices for temperature control of the heater, resulting in maintenance difficulties. Thus, the prior art system has been designed on the premise that the detailed maintenance or repair of the evaporation concentrator is practically impossible since the concentrator must be disposed in a cell or a glove box of limited space because of the radioactivity of the nitrate solution being treated. As a result, heavy restrictions have been inevitably imposed on the construction of the system, its control method, etc. Anyway, such a two-stage treatment system consisting of separate evaporation concentration equipment and denitrating equipment has been employed when nitrate solution must be treated, not on a laboratory scale, but in large quantities. Since each of these equipments must be operated as a separate equipment unit, not only is the working efficiency poor, but also the whole system must be enlarged, thereby reducing the operational energy efficiency.